Typically, optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element modules including optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. The optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units.
With such optical systems, the optical element units are often built from a stack of optical element modules holding one or more—typically rotationally symmetric—optical elements. The optical element modules usually include an external generally ring shaped support structure supporting one or more optical element holders each, in turn, holding one or more optical elements.